Han Hu, Gabriele Sansalone, Stephen Wroe, Paul G. McDonald, Jingmai K. OâConnor, Zhiheng Li, Xing Xu, and Zhonghe Zhou (2019)
Evolution of the vomer and its implications for cranial kinesis in Paraves
Proceedings of the National Academy of Sciences (advance online publication)
Cranial kinesis is an important feature of neornithine birds. However, due to the rarity of palatal elements in fossils, its origin is poorly understood. Here we reconstruct the vomer of the troodontid Sinovenator and the stem bird Sapeornis, and provide the palatal reconstruction of this Early Cretaceous bird. Using these data we conducted a comprehensive morphological study of the palate and a 3D shape analysis of the vomer across Paraves. The results indicate that stem birds, like palaeognaths, had limited cranial kinesis, a conclusion also supported by the identification of an ectopterygoid in Sapeornis. This suggests that the remarkably flexible avian skull is a neognathous innovation, with greater cranial kinesis ultimately facilitating the radiation of this highly successful lineage.
Most living birds exhibit cranial kinesisâmovement between the rostrum and braincaseâin which force is transferred through the palatal and jugal bars. The palate alone distinguishes the Paleognathae from the Neognathae, with cranial kinesis more developed in neognaths. Most previous palatal studies were based on 2D data and rarely incorporated data from stem birds despite great interest in their kinetic abilities. Here we reconstruct the vomer of the Early Cretaceous stem bird Sapeornis and the troodontid Sinovenator, taxa spanning the dinosaurâbird transition. A 3D shape analysis including these paravians and an extensive sampling of neornithines reveals their strong similarity to paleognaths and indicates that morphological differences in the vomer between paleognaths and neognaths are intimately related to their different kinetic abilities. These results suggest the skull of Mesozoic paravians lacked the kinetic abilities observed in neognaths, a conclusion also supported by our identification of an ectopterygoid in Sapeornis here. We conclude that cranial kinesis evolved relatively late, likely an innovation of the Neognathae, and is linked to the transformation of the vomer. This transformation increased palatal mobility, enabling the evolution of a diversity of kinetic mechanisms and ultimately contributing to the extraordinary evolutionary success of this clade.